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Arbuscular mycorrhizal (AM) fungi enhance plant stress tolerance, but it is unclear whether AM fungi affect heat tolerance in cucumbers. This study aimed to analyze how an AM fungus, Diversispora versiformis, affected growth, chlorophyll, five osmolytes, and plasma membrane intrinsic proteins (PIPs) and heat shock protein 70 (Hsp70) gene expression in cucumber leaves after a short-term (80 h) heat stress. Heat treatment significantly reduced root AM fungal colonization rate (0.26 folds). Heat treatment also distinctly suppressed plant height, stem diameter, and biomass, whereas AM fungal inoculation improved these growth variables as well as the chlorophyll index, with the benefit being more obvious under heat than under no-heat stress conditions. Heat treatment triggered differential changes in osmolytes (sucrose, fructose, and betaine) of inoculated and uninoculated cucumbers, whereas inoculation with AM fungus significantly raised leaf sucrose, fructose, glucose, betaine, and proline levels when compared to non-AM fungal inoculation. Heat treatment increased the expression of two (CsPIP1;6 and CsPIP2;1) of eight CsPIPs in inoculated and uninoculated plants, whereas AM fungal inoculation up-regulated the expression of CsPIP1;6, CsPIP2;1, and CsPIP2;6 under heat stress conditions. Hsp70s expressed differently in inoculated and uninoculated plants under heat versus no-heat stress, with 6 of 11 CsHsp70s down-regulated in inoculated plants. Under heat stress conditions, AM fungus only up-regulated CsHsp70-8 expression in 11 Hsp70s, while another eight CsHsp70s were down-regulated. Heat treatment and AM fungal inoculation both increased the expression of CsHsp70-8 and CsPIP1;6. It was concluded that AM fungus-inoculated cucumbers have high levels of growth, chlorophyll, and osmolytes under heat stress and do not require high CsPIPs and CsHsp70s expression to tolerate a short-term heat treatment. [ABSTRACT FROM AUTHOR]

• Whether and how soil AMF enhance heat stress tolerance of cucumber were studied. • Heat stress decreased root AMF colonization and soil hyphal length. • AMF increased cucumber biomass and leaf gas exchange under heat stress. • Most cshsp70s and cspips were up-regulated in inoculated plants under HS versus ST. • Under HS, AMF up-regulated expressions of 11 of 12 cshsp70s and 12 of 14 cspips. Cucumbers are frequently subjected to heat stress (HS) during the summer, which severely limits their growth and fruit yield and quality. Soil arbuscular mycorrhizal fungi (AMF) can enhance the abiotic stress tolerance of host plants, but it is unclear whether and how soil arbuscular mycorrhizal fungi (AMF) enhance the HS tolerance of cucumber. This work aimed to assess the effects of an arbuscular mycorrhizal fungus, Diversispora versiformis , on plant growth, leaf gas exchange, and expressions of plasma membrane intrinsic proteins (PIPs) and heat shock protein 70 (Hsp70) genes in roots of cucumber under HS (38 °C/30 °C, day/night temperature, 16 h/8 h) and suitable temperature (ST) (25 °C/18 °C, day/night temperature, 16 h/8 h) conditions. The HS treatment significantly decreased root mycorrhizal colonization and soil mycelium length by 0.26- and 0.55-fold, respectively, compared with the ST treatment. The biomass inhibition of cucumber by HS versus ST was more pronounced in uninoculated plants than inoculated plants. Inoculation with D. versiformis , however, significantly increased cucumber biomass and leaf gas exchange under both ST and HS conditions when compared to the uninoculated treatment. The HS treatment had almost no effect on or down-regulated the expression of CsHsp70s and CsPIPs genes in uninoculated plants, while most CsHsp70s and CsPIPs gene expressions were up-regulated in inoculated plants under HS versus ST conditions, suggesting the active heat resistance of mycorrhizal plants. Additionally, under ST conditions, D. versiformis up-regulated CsHsp70–11, CsPIP1;2 , and CsPIP2;6 expressions. Under HS conditions, D. versiformis increased the expression of 11 of 12 CsHsp70s by 1.17‒6.14 folds as well as 12 of 14 CsPIPs by 1.82‒144.43 folds, with CsPIP1.5 and CsPIP2;7 up-regulated up to 144.43- and 82.46-fold, respectively. It is concluded that D. versiformis -inoculated cucumbers exhibited higher heat tolerance than uninoculated plants, associated with up-regulated expression of most CsHsp70s and CsPIPs genes. [ABSTRACT FROM AUTHOR]